Determinants of tree mallow invasiveness

Invasive plants threaten global biodiversity and effective management strategies are required to control their aggressive spread. Multiple factors facilitate plant invasion, but the relative importance of these determinants remains unclear. Tree mallow, Lavatera arborea, provides a tractable system for defining the role and importance of a number of ecological factors in determining invasive behaviour. In this study, experimental field sites were established in Cornwall, England and around the Firth of Forth, Scotland to determine whether invasiveness is driven by tree mallow’s intrinsic characteristics or by attributes of the local environment (rabbit grazing, nutrient addition and physical disturbance). Litter samples collected from underneath tree mallow stands throughout the UK were sorted for seeds, seed remains and soil fauna to determine whether the relative contribution of soil fauna to tree mallow seed loss differed between invasive and non-invasive sites (i.e. sites invaded or not invaded by tree mallow). Tree mallow cover (%), population size, number of stems and mean height were used to distinguish between invasive and non-invasive sites and each site was analysed for soil nutrients. High tree mallow invasiveness (demonstrated by the competitive suppression of Festuca rubra) favoured low grazing, high nutrient availability and high disturbance. Soil fertility was a stronger determinant of tree mallow invasion success than disturbance but a combination of both promoted invasion and establishment. Non-native tree mallow plants within the UK were grazed less than congeneric natives. Soil fauna abundance on Craigleith Island and throughout the UK was negatively, but not significantly, associated with seed abundance and tree mallow cover. In conclusion, nutrient limitation appears as a means of preventing invasion and potential further study is discussed. Understanding the ecological factors in determining invasiveness is crucial for developing effective management strategies aimed at predicting and controlling invasion.